Sodium vapor lamp having an improved grooved alumina arc tube

ABSTRACT

The alumina arc tube of a high pressure sodium vapor arc discharge lamp has a spiral grove formed by raised shoulders on the outer surface thereof. A refractory metal wire heater is wound around the arc tube and seated in the groove in order to maintain uniform spacing between the turns of the heater throughout the life of the lamp.

[ Apr. 23, 1974 United States Patent [1 Zack et al.

313/225 X 313/15 313/225 X 313/225 X 313/15 X 3,439,209 4/1969 Ahsmann et al......... 3,721,846 3/1973 3,746,914 7/1973 3,757,158 9/1973 3,757,159 9/1973 Primary Examiner-H. K. Saalbach Mass.

Assistant ExaminerSiegfried H. Grimm Attorney, Agent, or FirmJames Theodosopoulos [73] Assignee: GTE Sylvania Incorporated,

Danvers, Mass.

Mar. 8, 1973 Appl. No.: 339,326

[22] Filed:

ABSTRACT [52] US. 313/15, 313/180, 313/220, The alumina arc tube of a high pressure sodium vapor 313/229, 313/344 arc discharge lamp has a spiral grove formed by raised [51] Int. H01j 7/24 shoulders on the outer surface thereof. A refractory 313/15, 25, 27, 180, 184, metal wire heater is wound around the arc tube and 313/220, 225, 228, 229, 344 seated in the groove in order to maintain uniform spacing between the turns of the heater throughout the life of the lamp.

[58] Field of Search.........

References Cited UNITED STATES PATENTS 2,765,416 Beese et 313/15 X 7 Claims, 2 Drawing Figures SODIUM VAPOR LAMP HAVING AN IMPROVED GROOVED ALUMINA ARC TUBE BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to are discharge lamps and, in particular, to high pressure sodium vapor lamps.

2. Description of the Prior Art Within the past few years, high pressure sodium vapor lamps have become commercially useful, especially for outdoor lighting applications, because of their high efficiency, generally in excess of 100 lumens per watt. The sodium operating vapor pressure in such lamps can vary from several millimeters to about 1000 millimeters Hg. Such lamps are called high pressure in order to distinguish them from low pressure sodium vapor lamps in which the sodium operating vapor pressure is in the order of a few microns. Low pressure sodium lamps have been in use for about thirty or forty years, but, although efficient, they produce an unattractive monochromatic yellow light. The color of light from high pressure sodium lamps is considerably improved over that from low pressure sodium lamps.

High pressure sodium lamps generally comprise an alumina ceramic arc tube and an arc tube fill of sodium, mercury and an inert gas. Examples of such lamps are shown in the following U.S. Pat. Nos: 3,248,590; 3,384,798; 3,448,319; 3,453,477; 3,485,343; 3,519,406; 3,521,108; 3,558,963 and 3,622,217.

One of the problems of high pressure sodium arc discharge lamps relates to the starting thereof. Such lamps require a considerably higher starting voltage to initiate an arc discharge than do other types of arc discharge lamps, such as fluorescent, mercury or metal halide. This higher starting voltage requirement necessitates the use of a special ballast for high pressure sodium lamps.

It is an object of this invention to provide a high pressure sodium vapor lamp having a reduced starting voltage, thereby eliminating the high voltage requirements of the ballast. Thus, a simpler more economical ballasting arrangement becomes quite practicable and, in fact, conventional mercury lamp ballasts can often be used with the lamps of this invention. This permits, in many cases, direct replacement of such mercury lamps by high pressure sodium lamps of this invention without any changes in the mercury lamp ballasts or fixtures.

SUMMARY OF THE INVENTION A high pressure sodium vapor lamp in accordance with this invention comprises an alumina arc tube having electrodes at its ends and containing a fill including sodium, mercury and an inert gas. A wire wound heater is disposed in heat transfer relationship with the arc tube in order to effect a substantial reduction in arc tube ignition voltage, as shown in co-pending application Ser. No. 214,000 filed December 30, 1971, now U.S. Pat. No. 3,746,914, assigned to the instant assignee, the disclosure of which is incorporated herein by reference.

In the instant invention, the wire wound heater is seated in grooves formed by raised shoulders spirally encircling the arc tube in order to maintain uniform turn spacing of the heater throughout the life of the lamp. Co-pending application Ser. No. 265,523, filed BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is an elevational view, partly in section, of a lamp in accordance with this invention.

FIG. 2 is an expanded sectional view of the arc tube of the lamp of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT A high pressure sodium vapor lamp in accordance with this invention comprises an outer glass envelope 1 which can be of elongated ovoid shape, such as is commonly used in high pressure sodium lamps, or of bulbous shape, such as is commonly used in mercury vapor and metal halide lamps. The neck of the envelope is closed by a reentrant stem 2 having a press 3 through which extend stiff lead-in wires 4,5 connected at their outer ends to a threaded shell 6 and center contact 7 of a conventional screw base.

Disposed within envelope 1 is an alumina arc tube 8, having raised shoulders 30 forming grooves 21, sealed at its lower end by niobium end cap 9 and at its upper end by niobium end cap 10. Niobium tubes 11,12 are brazed or welded to end caps 9,10 and are used to support are tube 8 as well as to conduct current to electrodes 13,14 within the arc tube. In addition, one of the niobium tubes serves as an exhaust tube during manufacture and is used to introduce a fill including an inert gas (e.g. argon or xenon), sodium and mercury into the arc tube, after which the niobium tube is sealed, such as by a cold weld.

Arc tube 8 is supported within envelope 1 by a structure consisting of side support wire 15, vertical support wire 16, metal strap 17 and horizontal metal strap 18. Support wire 15 is welded to lead-in wire 5, support wire 16 is welded to wire 15, strap 17 is welded to wire 16, strap 18 is welded to strap 17 and strap 18 is connected to upper niobium tube 12. Electrical connection between lead-in wire 5 and upper electrode 14 is established by said structure.

The lower end of arc tube 8 is supported by metal strap 19 which is connected between niobium tube 11 and support wire 20, with support wire 20 being welded to lead-in wire 4. Electrical connection is also established thereby between lead-in wire 4 and lower electrode 13.

Encircling arc tube 8 and seated in groove 21 is a wire wound heater 22 of the type disclosed in copending application Ser. No. 214,000. The upper and lower ends of heater 22 are electrically connected, re spectively, to the lower and upper electrodes in order to further reduce the lamp starting voltage, as disclosed in co-pending application Ser. No. 266,294, now U.S. Pat. No. 3,721,846.

The upper end of heater 22 is connected to a wire 23 which is embedded in glass rod 24. Glass rod 24 is supported from side wire by two wires 25 which are welded to side wire 15 and embedded in glass rod 24. Also embedded in glass rod 24 is another wire 26 one end of which is connected to support wire by a thin wire 27. A U shaped bimetallic switch 28 makes electrical connection between wire 23 and wire 26. The result of this electrical arrangement is that the voltage applied to lower electrode 13 is also applied to the upper end of heater 22. Switch 28 has a predetermined opening temperature which exceeds the temperature at which arc tube ignition occurs. Upon opening switch 28, heater 22 is electrically removed from the circuit.

The lower end of heater 22 is electrically connected to side support wire 15 by a similar arrangement of wires 23, 25' and 26, glass rod 24' and switch 28'. Switch 28' also opens after arc tube ignition to electrically isolate heater 22.

Attached to the upper and lower ends of side wire 15 are spring fingers 31 which press against the interior wall of envelope 1 and provide increased support for the arc tube. Also disposed on the lower end of side wire 15 are ring getters 29, since envelope 1 contains a vacuum to reduce arc tube heat losses.

Shoulders which form grooves 21 of arc tube 8 are raised, instead of being machined into the arc tube wall, in order to avoid weakening of the arc tube at the grooves. For maximum transmission of light, the wall of arc tube 8 should be quite thin but not so thin that arc tube 8 fails to maintain dimensional stability throughout the life of the lamp or that the arc tube filling material can escape through the wall. We have found that such grooves can be simply and inexpensively formed at the time of pressing alumina powder into a cylindrical tube which is subsequently processed into arc tube 8. The cylindrical tube is prepared by isostatically pressing high purity, sub-micron particle size alumina powder, containing a small amount of material for controlling grain growth such as magnesia, at a high pressure, such as 12,500 psi.

The mold for the cylindrical tube is made of a hard rubbery material, such as polyurethane, and is itself a thick walled cylinder. On the inner surface of the mold there are recessed spiral grooves, the purpose of which is to form raised shoulders on arc tube 8. For manufacturing convenience, said recessed spiral grooves are radiused; this results in radiused shoulders 30 on arc tube 8, groove 21 being the channel between shoulders 30.

A metal rod, the purpose of which is form the bore of arc tube 8, is axially positioned within the mold and the space between the rod and mold is filled with alumina powder. Vibrating is used to ensure thorough filling, especially in the grooves of the mold. After isostatic pressing, the cylindrical tube of pressed alumina powder is removed from the mold. There is sufficient shrinkage of the powder during pressing so that the raised shoulders easily clear the inside diameter of the mold. For example, for an arc tube used in a 130 volt 360 watt lamp, the inside diameter of the mold was 585 mils and the depth of the grooves was 30 mils. The outside diameter of the pressed alumina tube was 474 mils and the height of the raised shoulders was about 20 to 25 mils.

The pressed alumina tube was fired at 1050C for 1% hours. Maximum densification and strength was then obtained by firing the alumina tube at l700C., which shrank the diameter of the arc tube to 350 mils and the height of the raised shoulders to 16 mils. The tube was then cut to an accurate length. The wall thickness of arc tube 8 was 31 mils.

As shown in FIG. 2, heater 20, which in this example was a coiled coil made of 4880 mm of 2% mil tungsten wire having a primary coiled diameter of 17 mils, was wound between a pair of parallel raised shoulders 30 which formed groove 21. The width of each shoulder 30 was about 20 mils, the space between shoulders was 26 mils and the height of each shoulder was 16 mils. There were a total of 14 secondary turns of wire uniformly extending along an intermediate 3 inch length of arc tube 8 which had a total length of 4 inches. The arc length or distance between electrodes 13 and 14 of this lamp was 3 7/16 inches. In order to ensure that heater 20 remains seated in groove 21, the depth of the groove should be greater than about 60 percent of the secondary diameter of the heater wire. In normal operation, heater 20 consumed about watts of power and heated arc tube 8 to are ignition in about two minutes.

We claim:

1. A high pressure sodium vapor lamp comprising: a glass envelope; an alumina arc tube disposed within said envelope and having electrodes sealed therein at the ends thereof and containing a filling including sodium, mercury and an inert gas, said are tube having a spiral groove formed by raised shoulders on its outer surface extending from about one electrode to the other, the shoulders defining said groove being raised above the outer surface of said are tube; and a heater of refractory metal wire wrapped around said are tube and seated in said groove.

2. The lamp of claim 1 wherein said groove is a channel between two parallel raised shoulders on said outer surface.

3. The lamp of claim l'comprising, in addition, a thermal switch, one end of said heater being electrically connected to the electrode at the opposite end of said arc tube through said thermal switch, said switch being normally closed during non-operation of said lamp and opening after ignition of the arc tube and remaining open during normal lamp operation.

4. The lamp of claim 2 wherein said heater is a coiled coil.

5. The lamp of claim 4 wherein the depth of said channel is greater than about 60 percent of the primary diameter of said heater.

6. The lamp of claim 2 wherein said shoulders are radiused.

7. The lamp of claim 5 wherein the width of said channel is less than double the primary diameter of said heater. 

1. A high pressure sodium vapor lamp comprising: a glass envelope; an alumina arc tube disposed within said envelope and having electrodes sealed therein at the ends thereof and containing a filling including sodium, mercury and an inert gas, said arc tube having a spiral groove formed by raised shoulders on its outer surface extending from about one electrode to the other, the shoulders defining said groove being raised above the outer surface of said arc tube; and a heater of refractory metal wire wrapped around said arc tube and seated in said groove.
 2. The lamp of claim 1 wherein said groove is a channel between two parallel raised shoulders on said outer surface.
 3. The lamp of claim 1 comprising, in addition, a thermal switch, one end of said heater being electrically connected to the electrode at the opposite end of said arc tube through said thermal switch, said switch being normally closed during non-operation of said lamp and opening after ignition of the arc tube and remaining open during normal lamp operation.
 4. The lamp of claim 2 wherein said heater is a coiled coil.
 5. The lamp of claim 4 wherein the depth of said channel is greater than about 60 percent of the primary diameter of said heater.
 6. The lamp of claim 2 wherein said shoulders are radiused.
 7. The lamp of claim 5 wherein the width of said channel is less than double the primary diameter of said heater. 